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Help me find the VCO for a CPLD

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Newbie level 4
Jun 6, 2007
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Hi i have designed a PCB with a CPLD on it and i am
really having trouble finding VCO(voltage controlled oscillators) for the CPLD.

I want to feed the CPLD (Max II) with a clock 10Mhz.
Isn't any other way to avoid using a VCO.
I have seen different schematics for CLPD and FGPA and they all
use VCOs.

Thanks in advance guys:)

Re: Clock input CPLD

You can use normal TTL oscillators that are very widely available and relatively cheaper than VCOs. You can also use normal quartz crystals with a little feedback circuit consisting of resistors,capacitors and a not gate.

Normally you won't be needing a VCO unless you want to change your clock frequency. In which case you will have to use one., they can ship to anywhere in the world for very little shipping costs. Check with your local electronic parts retailer, he may be having some selections.

Clock input CPLD

As kishor24k suggested, most CPLD and FPGA projects use an ordinary fixed-frequency crystal oscillator (XO), not a Voltage Controlled Oscillator (VCO).

Be sure to buy an oscillator that has a compatible output voltage for your CPLD input.

If you can't find a suitable part near you, DigiKey in USA also stocks many types, and ships to many countries:

Re: Clock input CPLD

echo47 I live in Europe and i cant find anyone to give me a fixed-frequency crystal oscillator (XO). I thought VCO and XO where the same sorry. I'm trying to avoid digikey for now beacuse the cheapest shipping fee for Greece is about 40$.Farnell which is in UK doesnt have them.They sent me to Newark.

kishore2k4 u wrote that i could use normal quartz crystals with a little feedback circuit consisting of resistors,capacitors and a not gate. Can u show me some schematics?
I dont wont to change the frequency. As i said above i just want it to be 10Mhz. Nothing more.

Thank you

Re: Clock input CPLD

**broken link removed**, The circuit can be scaled to 10Mhz with CMOS/HCMOS Inverter. The oscillator costs $1.9 and standard shipping charges to anywhere in the world is only $5, so if you want something stable and reliable then definetly go for the latter.


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